Used 04 Prius for sale -- 15K miles w/o oil change -- how bad?

Looks like seller had oil changed every 5K miles by the dealer except for once it went 15K (9 months) with no service (based on dealer maintainence record) -- I know that's not good but if all other oil changes were as recommended by Toyota (5K) and I am scrupulous with future maintainence, what kind of life expectancy does the engine probably have? I'd like it to last past 200K at least, it has 50K currently and I'll be driving 100 miles a day every day....